As described in commonly owned U.S. Pat. Nos. 5,628,220 and 5,765,420 of Schafer is it known to bulge-form a workpiece, normally a tube, by fitting it to a die and then internally pressurizing the workpiece to such a high pressure that it is deformed outward to take the shape of the tool or die. The result is a very strong cold-formed piece whose shape perfectly corresponds to that of the die.
It is often necessary to punch a hole in such a workpiece. Rather than do this as a separate step from outside, it is known from German 195 06 067 of Herberg and from U.S. Pat. No. 5,460,026 of Schafer to provide the shaping tool with a punch assembly comprising a guide sleeve whose inner surface is flush with an inner surface of the tool cavity and a punch fitting complementarily in the guide sleeve and movable between an inner position projecting into the tool slightly past the sleeve and an outer position recessed back in the sleeve.
During the initial shaping operation the punch is advanced slightly to its inner position to produce an inwardly raised region of the same shape of the punch and having an outer periphery defined by a line of reduced thickness where the material of the workpiece is stretched over the edge of the punch. The punch is then retracted and, if necessary, the pressure in the workpiece is increased to drive out the piece and cut it off at the weakened line. This procedure is extremely convenient in that it is done right in the shaping tool during the initial formation operation, and the cut-out piece ends up outside the workpiece.
The disadvantage of this system is that the subassembly formed by the punch and its guide sleeve must be meticulously mounted in the tool and secured very solidly in place. This is typically done by bolting a holding plate over the outside of these parts. Thus when they need to be changed, it is necessary to remove a relatively bulky holding plate and then pull out the sleeve and punch.